Short answer: The receiving server deliberately deferred your message because it hasn't seen you before, and asks you to retry in a few minutes. This is called greylisting; standards-compliant servers retry automatically and the second attempt is accepted.
451 4.7.1 bounce message examples
The same code comes with different wording on different servers. In the bounce (non-delivery report) you may see one of these lines:
451 4.7.1 Greylisting in action, please come back later 451 4.7.1 Please try again later 451 4.7.1 <jane@example.com>: Recipient address rejected: Greylisted 451 4.7.1 Temporary local problem - please try later
Type: Temporary error. The sending server keeps the message in its queue and retries automatically.
What causes it?
| Cause | Explanation |
|---|---|
| Greylisting | The receiver temporarily rejects the first message from a new combination of sending IP, sender address and recipient address. Most spam bots never retry, so this stops a significant share of spam. |
| Rate limiting | Senders opening too many connections in a short time are temporarily slowed down. |
| Temporary problem on the receiving side | A component such as the disk, database or content scanner isn't responding at the moment. |
What to do as the sender
- Usually you don't need to do anything: your server will retry, typically within 5-30 minutes, and the message will be delivered.
- If mail stays undelivered for hours, check your server's retry settings; some web applications and simple SMTP libraries don't retry on 4xx.
- Services that send from many different IPs (large cloud providers) can be delayed longer by greylisting; that's normal.
What the receiving side's admin should check
- If you use greylisting, allow-list the IP ranges of large providers (Google, Microsoft) and partners you exchange mail with often.
- If first-message delays bother your users (for example one-time sign-in codes), rely more on content and reputation filtering instead of, or alongside, greylisting.
Good to know
Greylisting is one of the oldest and cheapest layers a spam filter uses: without looking at content, it removes part of the spam just by asking "will you retry?". Its only downside is a few minutes' delay on the first message. Modern email security gateways combine it with blocklists, SPF/DKIM/DMARC checks, content analysis and virus scanning.
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Frequently asked questions
Will greylisting lose my email?
No. Every standards-compliant mail server keeps the message in its queue and retries on a 4xx response. The mail is only delayed.
Verification code emails arrive late. Could greylisting be the reason?
Yes. A server seeing the sender for the first time may hold the message for a few minutes. Allow-listing the service that sends the codes on the receiving side fixes it.